[ci] Share bazel verilator model build between jobs

CI currently runs two parallel jobs to run the fast and slow verilator tests on earlgrey. Each job will first build the Verilator model for the chip before running the tests. It should be possible to avoid this duplicate work by using one job to just build the Verilator model and upload it as an artifact. Then all subsequent test jobs can download that model and run tests. Alternatively, it may be possible to just upload the entire Bazel local cache instead of trying to locate and place the Verilator model itself.

Even though we have a remote cache, PRs cannot write to it which necessitates this mechanism to transfer artifacts between jobs within the same PR.